4 Consider, Brethren, this “one thing,” and see if even in multitude itself anything pleases, but “this oneness.” See how great a number, through God's mercy, you are: who could bear you, if you did not mind “one thing”? Whence in this many is this quiet? Give oneness, and it is a people; take oneness away, and it is a crowd. For what is a crowd, but a disordered multitude? But give ear to the Apostle: “Now I beseech you, brethren.” He was speaking to a multitude; but he wished to make them all “one.”
“Now I beseech you, brethren, that you all speak the same thing, and that there be no schisms among you; but that you be perfected in the same mind, and in the same knowledge.” And in another place, “That ye be of one mind, thinking one thing, doing nothing through strife or vainglory.” And the Lord prays to the Father touching them that are His: “that they may be one even as We are One.” And in the Acts of the Apostles; “And the multitude of them that believed were of one soul, and of one heart.” Therefore, “Magnify the Lord with me, and let us exalt His Name in one together.” For one thing is necessary, that celestial Oneness, the Oneness in which the Father, and the Son, and Holy Spirit are One.
See how the praise of Unity is commended to us. Undoubtedly our God is Trinity. The Father is not the Son the Son is not the Father, the Holy Spirit is neither the Father, nor the Son, but the Spirit of both; and yet these Three are not Three Gods, nor Three Almighties; but One God, Almighty, the whole Trinity is one God; because One thing is necessary. To this one thing nothing brings us, except being many we have one heart.
Source: Sermons on Selected Lessons of the New Testament (New Advent)